BAGRAM AIRFIELD, Afghanistan—Afghan soldiers returned to their barracks to prepare for dinner last Friday when all the generators suddenly fell silent and the lights on this massive base, the centerpiece of America’s war effort in Afghanistan, shut off.

“It just went dark,” said Sgt. Ehsanullah, an Afghan soldier stationed there who uses only one name. Once the power went out, the water stopped pumping, too, he and other Afghan soldiers here said. Scores of Afghan civilians entered the facility and looted it, stealing supplies before being turned away.

The Americans’ departure—the exact timing of which had been kept secret for security reasons, despite weeks of preparations alongside the Afghan military—came just as the Taliban have been advancing across the country, including in the vicinity of Bagram, about 40 miles northeast of Kabul. While the U.S. military has made public its intention to leave, and worked with Afghan officials on the handover, the departure still came as a shock to many Afghan soldiers, leaving many rattled.

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Propaganda footage appears to show Taliban forces guarding a U.S.-built border crossing between Afghanistan and Tajikistan. WSJ’s Gordon Lubold reports from Kabul as American troops withdraw and the Taliban claim control of more than one third of the country. Photo: Sky/Associated Press The Wall Street Journal Interactive Edition

U.S. Army Col. Sonny Leggett, a spokesman for the U.S. military command, said that the transfer of the Bagram airfield to the Afghans was completed after an extensive process over several weeks, adding that all handovers of Bagram and other bases had been carried out in close coordination with the Afghans.

Now, Afghan officials are trying to figure out how they will protect the base, maintain it—and turn the lights back on. The U.S. will continue to support and fund Afghanistan’s security forces, but helping Afghans from afar, such as with restarting generators, for example, will be a challenge.

“We are Afghan, we have to solve our problem, we have to secure our country, and once again build our country by our own hand,” said the base’s new Afghan commander, Brig. Gen. Mir Assadullah Kohistani.

In addition to taking over the deserted base, Gen. Kohistani has received an estimated 3.5 million items that U.S. officials had said they were leaving behind. That inventory includes hundreds of cars and trucks, some weaponry and ammunition.

Sgt. Ehsanullah, squatting on an American armored vehicle on Monday at Bagram airfield, said the base ‘just went dark’ after the last U.S. forces withdrew.

Sgt. Ehsanullah, squatting on an American armored vehicle on Monday at Bagram airfield, said the base ‘just went dark’ after the last U.S. forces withdrew.

But there are also hundreds of thousands of items that can best be described as the detritus from two decades of America’s military involvement: flagpoles, desk chairs, telephones, pallets of plywood, tents, refrigerators, dollies and boxes of Dr Pepper soft drinks.

Afghan officials said they don’t know what to do with much of the American stuff they now own. But Gen. Kohistani said he appreciated some of the remaining infrastructure, including a 50-bed hospital, fuel storage tanks, other military and civilian equipment and radars for the flight operations. The base also houses a prison that is under Afghan control and still holds about 5,000 inmates, he said, most of them suspected members of the Taliban or Islamic State.

The massive airfield, which served as the hub of the Soviet military occupation in the 1980s, was appropriated by the U.S. once American Green Berets landed in Afghanistan weeks after the Sept. 11, 2001, attacks that al Qaeda had plotted on Afghan soil.

President Biden directed in April that all American troops must come out of Afghanistan by Sept. 11 this year. Since then, nearly a third of Afghanistan’s districts have fallen to the Taliban as Afghan forces, deprived of American air support that often came from Bagram, surrendered en masse. A recent U.S. intelligence assessment says Afghanistan could fall to the Taliban within six months of the American forces leaving.

Already, the consuls of the four nations that maintain diplomatic missions in the northern Afghan city of Mazar-e-Sharif—India, Pakistan, Iran and Turkey—have left for Kabul, said Mohammad Afzal Hadid, the provincial council chief. Junior consulate employees remain in the city, he said.

The Parwan province where Bagram is located is also coming under increasing Taliban attacks. On Monday, Gen. Baba Jan, a local strongman and former anti-Soviet commander in the area, came to visit the facility, accompanied by dozens of tribal chiefs, and pledged to defend Bagram from the Taliban.

Brig. Gen. Mir Assadullah Kohistani, the new Afghan commander of the Bagram base, greeted tribal leaders in his office at the base on Monday.

Brig. Gen. Mir Assadullah Kohistani, the new Afghan commander of the Bagram base, greeted tribal leaders in his office at the base on Monday.

“We will take care of the security outside of the base, if you take care of the security inside,” he told Gen. Kohistani, putting a traditional chapan coat on his shoulder and offering congratulations.

The Afghan military is likely to struggle with maintaining Bagram, a vast facility that requires extensive know-how and funds to keep running. In accordance with the agreements that President Donald Trump reached with the Taliban last year in Doha, Qatar, the U.S. military withdrawal also includes the pullout of civilian contractors who serviced the coalition’s bases.

On Monday, a late-model black Chevrolet Suburban, a common must-have vehicle for U.S. senior officers, sat on the side of a road in the middle of Bagram with a big yellow ‘X’ on its windshield. A white school bus was parked on another road, its rear door still open. More than a dozen Mine-Resistant, Ambush-Protected vehicles, or MRAPs, sat in the corner of a weedy field with a lone Afghan soldier guarding them.

Hundreds of cars, trucks and buses were left at the base by the U.S. military.

Hundreds of cars, trucks and buses were left at the base by the U.S. military.

A lot of the Americans’ excess material was destroyed or trashed, and some equipment was shipped out of the country. Still, a lot more has stayed behind.

A new fire tanker truck stood alone in a parking lot on Monday. Old bunkers, warehouses, hangars, tents, shipping containers and light towers pepper the base. Miles of the 20-foot cement barriers lined the roads. One of the base’s two runways, built in 2006 at an estimated cost of $96 million to American taxpayers, is empty. A package of brown cinnamon Pop-Tarts, a staple of U.S. military cafeterias, sat on a dusty filing cabinet covered with stickers to identify these cabinets as once containing classified information.

Sgt. Ehsanullah’s new job, for now, is to guard one of the parking lots filled with all the trucks, buses and other vehicles the Americans left behind. The lot contains Chevrolet and Ford trucks, Land Rover Defenders supplied by the British government, and numerous buses, once used to ferry troops from one side of the base to another. Few appear to have any keys in them, and Sgt. Ehsanullah just shrugged when asked what the Afghans would do with all the vehicles.

“We are the guards of this place,” he said. “We don’t know what they are going to do with these cars.”

Afghan National Army soldiers are now responsible for guarding the base and the equipment on it.

Afghan National Army soldiers are now responsible for guarding the base and the equipment on it.

Write to Gordon Lubold at Gordon.Lubold@wsj.com